After hydrocarbons have been removed from the ground, the fluid stream (such as crude or natural gas) is transported from place-to-place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Ultrasonic flow meters may be used to measure the amount of fluid flowing in a pipeline, and ultrasonic flow meters have sufficient accuracy to be used in custody transfer.
In an ultrasonic flow meter, acoustic signals are sent back and forth across the fluid stream to be measured. In fact, many ultrasonic meters have multiple transducer pairs, each transducer of a transducer pair both sending acoustic signals and receiving acoustic energy. While in some situations one or both transducers of a transducer pair may cease to operate instantly, in most cases failure of a transducer is over time, with the performance of the transducer (and therefore the pair of transducers considered together) degrading until insufficient information can be gleaned from the transducer pair. For example, signal-to-noise ratio of electrical signals created in response to acoustic energy incident on a transducer may decrease as performance degrades for a transducer.
Environmental factors also affect performance of transducers even in situations where a transducer pair is operating properly. In an illustrative case of signal-to-noise ratio, environmental factors such as pressure of the fluid in a meter, type of fluid in the meter, position of the valve member of upstream control valves, and condition of an upstream flow straightners all affect signal-to-noise ratio. When performance degradation is noted over a course of days, weeks or months, it may be difficult to determine whether the performance degradation is attributable to forthcoming failure of transducers, or changes in environmental factors.